The present invention relates to a smart antenna method and apparatus and, more particularly, but not exclusively to the use of polarization as an additional parameter for determining the angle of arrival of impinging electromagnetic radiation.
Vector sensors enable estimation of the angle of arrival and polarization of impinging electromagnetic waves with arbitrary polarization. In the last decade, many array processing techniques for source localization and polarization estimation using vector sensors have been developed. A. Nehorai and E. Paldi, “Vector-sensor array processing for electromagnetic source localization,” IEEE Trans. on Signal Processing, vol. 42, pp. 376-398, February 1994, the contents of which are hereby incorporated by reference, developed the Cramer-Rao bound (CRB) and the vector cross-product direction-of-arrival (DOA) estimator. Polarimetric modeling using vector sensors is performed in B. Hochwald and A Nehorai, “Polarimetric modeling and parameter estimation with application to remote sensing” IEEE Trans. On Signal Processing, Vol. 43 pp. 1923-1935, August 1995 the contents of which are hereby incorporated by reference.
Identifiability and uniqueness issues associated with vector sensors are analyzed in G. F. Hatke, 27th Asilomar Conf. 1993, pp 1365-1369, K. C. Ho, K. C. Tan, W. Ser, Signal Processing Vol. 47 pp 41-54 November 1995, B. Hochwald and A Nehorai, IEEE Trans on Signal Processing, Vol. 44 No. 1 pp. 83-95 January 1996, K. C. Tan, K. C. Ho, A. Nehorai IEEE Trans. Signal Processing, Vol. 44 pp. 3099-3107, December 1996. The contents of each of the above documents are hereby incorporated by reference.
Eigenstructure-based techniques, such as ESPRIT and multiple signal classification (MUSIC) for source localization using vector sensors have been extensively investigated. J. Li, IEEE Trans. Antenna Propagation. Vol. 41. pp. 379-387 March 1993, the contents of which are hereby incorporated by reference, applied the ESPRIT algorithm to a vector sensor array. ESPRIT-based direction finding algorithms using vector sensors have been further investigated in several papers including K. T. Wong and M. Zoltowski, IEEE Trans. Antenna Propagation. Vol. 48 pp 671-681, May 2000, the contents of which are hereby incorporated by reference. MUSIC-based algorithms for the same problem have been applied in K. T. Wong and M. Zoltowski, IEEE Trans. Antenna Propagation. Vol. 48 pp 1235-1245 and 2205-2210, August 2000. These techniques yield high-resolution and asymptotically efficient estimates in case of uncorrelated or partially correlated signals. However, since these techniques assume a non-singular signal correlation matrix, they encounter difficulties in cases of fully correlated signals such as signals from a single source in a multipath scenario. The latter is particularly common in cellular telephony in the urban environment.
In order to decorrelate signals in a data covariarice matrix, Evans et al. in Proc. 1st ASSP Workshop spectral Estimation, Hamilton Ontario Canada, 1981 pp 34-139, the contents of which are hereby incorporated by reference, proposed a preprocessing technique referred to as spatial smoothing. Several later authors investigated the spatial smoothing method, in combination with the method of forward-backward averaging. The drawback of the above combination is the reduction of the effective array aperture length, resulting in lower resolution and accuracy. An alternative spatial averaging method is redundancy averaging. It has been shown that a preprocessing method based on redundancy averaging induces bias in the DOA estimates.
A Maximum Likelihood (ML) approach for diversely polarized source localization was proposed in Wax and Ziskind, IEEE Transactions Antenna Propagation vol 38, pp. 111-1114, July 90. The maximum likelihood method uses the simulated annealing algorithm to efficiently search over the signal DOA's and polarization vectors.
The above approach however still does not overcome the difficulties introduced by having correlated signals, as would be expected when seeing multi-path versions of the same signal.
There is thus a widely recognized need for, and it would be highly advantageous to have, a decorrelation method usable with a smart antenna, which is devoid of the above limitations and is therefore useful for cellular telephony in urban areas and for other cases where multi-path is a significant problem.